This invention relates to an apparatus for recognizing whether a field is an odd line field or an even line field of an interlaced composite video signal. More particularly, this invention relates to an apparatus for multiplexing two video signals at the field rate or decoding the multiplexed signal at the field rate (e.g. for use with signal associated with a stereoscopic television system employing alternate fields for left and right stereoscopic images and employing a PLZT viewer). The stereoscopic television system employing alternate fields for left and right eye images, which is the principal object of this invention, needs synchronization between the left and right eye images and the viewer's shutter operation. If the system falls out of sync so that the left eye image is shown to the right eye and the right eye image is shown to the left eye, the image perceived is confusing and not truly stereoscopic.
In most conventional methods of synchronization, the driving pulse of the viewer is triggered by a T-type flip-flop which is triggered by the vertical sync pulse derived from the composite video signal, but there is no way to define which field is which. In another method of synchronization, a discernible signal which is inserted in the vertical interval of the video signal prevents free editing of a video tape storing such a signal because of the fact that the discernible signal fades away after editing.
Yet another method of synchronizing by coincidence of the horizontal sync pulses with the pulses derived from the vertical sync signal cannot be expected to provide stable operation because of the critical adjustment required and its dependence upon temperature. (c.f. U.S. Pat. No. 4,145,713)
Therefore, there is a continuing need for an improvement for a stereoscopic television system which assures the identity of the odd and even line field.